Optical coherence tomography (OCT) systems are known as being useful for producing high resolution and three-dimensional images of samples. Swept source optical coherence tomography (SS-OCT) systems are a subclass of OCT systems using a frequency-sweeping light source.
SS-OCT systems, such as the ones depicted in FIG. 1 (PRIOR ART), typically use a sweeping tunable laser to interrogate an optical interferometer. The sweeping tunable laser emits a laser beam towards the interferometer. The laser beam passes through a beam splitter and is separated into a sample beam and a reference beam. The sample beam (solid line) reaches and illuminates the sample (schematically illustrated as one individual scatterer on FIG. 1), and then returns back to the beam splitter before being reflected towards an optical detector. The reference beam (dashed line) propagates along the reference arm and is reflected by a plane reference mirror back towards the detector. The reference beam and the sample beam interfere on the photosensitive surface area of the detector, producing an electrical signal herein referred to as the “analog OCT signal”. Due to the optical path length difference between the sample beam and the reference beam, and due to the sweeping of the optical emission frequency, the optical frequencies of the two beams reaching the detector differ by an amount 2×m/c, where x is the optical path length difference, m the rate of change of the optical frequency and c the speed of light. The interference of these two beams on the detector will result in the OCT electrical signal at the difference of the two optical frequencies (associated to the reference beam and the sample beam—also called the “beat frequency” in the case of an individual scatterer).
A typical sample is however formed of multiple individual scatterers, and each individual scatterer illuminated by the sample beam contributes to the OCT electrical signal. Indeed, each individual scatterer is associated with its own optical path length difference and its corresponding beat frequency, so that interferometric beam is the result of the coherent sum of all those frequencies. The OCT electrical signal is therefore an analog representation of the optical interferometric beam envelope which correlates to the sample reflectivity profile along the sample beam propagation direction through a Fourier transform operation after applying the proper sweep nonlinearity corrections.
In many applications, SS-OCT systems may image a few hundred of microns within the sample (i.e., below the surface of the sample). This limitation is not imposed by the imaging range of the SS-OCT systems, but rather by the light attenuation within the sample. Indeed, even though an OCT system may have an imaging range of several millimeters or centimeters, the signal reflected by the sample is only coming from a section along the optical axis (typically a fraction of a millimeter) within the larger imaging range. This situation is depicted in FIG. 2.
FIG. 2 illustrates an interferometer configuration that may be included in an SS-OCT system. The situation depicted in FIG. 2 is one where the imaging range of the system is substantially larger than the penetration depth of the light within the sample. FIG. 2 also displays at line B a diagram illustrating the reflectivity profile of the sample along the optical axis (i.e., along the sample beam propagation direction). The light attenuation profile of the sample beam along the optical axis is also illustrated at line C, wherein the intensity of the sample beam with respect to the distance travelled through the sample is clearly represented. Line D shows the corresponding OCT electrical signal's spectrum. One skilled in the art would understand that, in the scenario depicted in FIG. 2, the center frequency of the OCT electrical signal, which could be, in some scenarios, in the RF range, is relatively high, while its bandwidth is relatively narrow (compared to the value around which is centered the OCT electrical signal).
It remains a challenge to measure samples presenting surface variations that exceed the imaging range of the SS-OCT systems of prior art. There is thus a need for a system for imaging such samples.